“This terrain is not easy to explain,” said Jeff Moore, the head of the New Horizons geology team at NASA’s Ames Research Center in California.

When astronomers focused on the “heart of Pluto,” New Horizons’ Ralph instrument revealed evidence of carbon monoxide ice. The contours highlight that the concentration of frozen carbon monoxide increases towards the center of the “bull’s eye.”

However, what was mind-boggling for scientists was the absence of craters on Pluto. They said that suggests to their surprise that Pluto is geologically active even now, and is being sculpted not by outside forces but by internal heat.

They speculated at a news conference at Cape Canaveral, Florida, Friday that Pluto is still being shaped by active geological processes.

Moore said scientists are still in the most preliminary stages of the investigations into Pluto’s surface.

The nuclear powered New Horizons – moving faster than any spacecraft ever built at a speed of more than 49,500 kilometers per hour – sped by Pluto Tuesday, getting as close as 12,500 kilometers and grabbing a huge volume of data.

The New Horizons mission officially completes man’s journey to the very edges of the solar system – and also makes the US the first country to successfully send a space probe to every planet from Mercury to Pluto.